In order to brake an automobile, a disc brake is widely used. In braking by the disc brake, a pair of pads arranged on both sides in an axial direction of a rotor that rotates together with a wheel are pressed on both sides of the rotor by a piston. As such the disc brake, disc brakes of various structures have been known. An opposed-piston type disc brake in which pistons are provided on both sides of a rotor in the opposed state to each other, can obtain stable braking power. Therefore, application of this opposed-piston type disc brake is increasing recently.
FIG. 10 shows such the opposed-piston type disc brake described in JP-U-05-027364. In an opposed-piston type disc brake 1, a caliper 5 composed of an outer body part 3 and an inner body part 4 is provided in a position where it interposes a rotor 2 between, and an outer cylinder and an inner cylinder are provided in these respective body parts 3 and 4 in a state where their respective opening parts are opposed to each other through the rotor 2. Into these outer cylinder and inner cylinder, an outer piston and an inner piston are fitted liquid-tightly and movably in the axial direction of the rotor 2. Further, in the outer body part 3, an outer pad is supported movably in the axial direction of the rotor 2; and in the inner body part 4, an inner pad is supported movably in the axial direction of the rotor 2. In the braking operation, pressure oil is fed into the outer cylinder and the inner cylinder, and the outer pad and the inner pad are pressed on both sides of the rotor 2 by the outer piston and the inner piston. In case of the structure described in JP-U-05-027364, the outer body part 3 and the inner body part 4 that are formed separately from each other are coupled by plural joint bolts thereby to form the caliper 5.
Further, in JP-A-2001-107994 and JP-A-01-210628, an opposed-piston type disc brake in which an outer body part and an inner body part that constitute a caliper are formed integrally is described. In the structure described in JP-A-2001-107994, the caliper is integrally formed by casting light alloy such as aluminum alloy, or alloy of iron family. In case that the assembly working efficiency of the opposed-piston type disc brake is taken into consideration, the structure in which the caliper is thus integrally formed does not require labor of coupling and fixing the outer and inner body parts to each other, compared with the conventional structure shown in FIG. 10.
The disc brake caliper is provided on a wheel side than a spring in a suspension system disposed between a vehicle body and the wheel. So, the disc brake constitutes a part of unsprung weight. Therefore, even small increase in weight of the disc brake has a large influence on driving performance centered at driving stability and fuel consumption. Therefore, from a viewpoint of improving the driving performance, it is preferable that the caliper is made of lightweight aluminum alloy. However, the caliper is easy to become high in temperature in the braking operation. Namely, in the braking operation, by the friction produced between the pad and the rotor, dynamic energy is converted into heat energy. This heat energy, for example, in the following forms, is transmitted to the caliper and brake oil.    (1) Conduction: Heat is transferred from the pad to the caliper coming into contact with the pad, or to the caliper and the brake oil through the piston.    (2) Radiation: Heat is transferred from the surfaces of the rotor and the pad to the caliper as an electromagnetic wave (radiation heat), increases the temperature of this caliper, and is transferred to the brake oil.
The heat energy is transmitted to the caliper and the brake oil in such the forms, so that the temperatures of these caliper and brake oil increase. Particularly, in case of the aluminum alloy caliper, this temperature increase becomes remarkable. For example, the temperature of caliper, in the braking operation, frequently exceeds 200° C. As this temperature increase of the caliper becomes remarkable, the temperature increase of the brake oil also becomes remarkable, which impedes occasionally good braking performance from being secured. Repeat of the remarkable temperature increase and decrease of the caliper causes the strength of this caliper to lower early.
In addition to the above Patent Documents, JP-A-2003-028219 also discloses related art.
The invention, in view of the above circumstances, aims at the heat transmission in the form (2) of the aforesaid forms (1) and (2), and has been made in order to reduce the temperature increase of a caliper and a brake oil due to heat generation produced by friction between a rotor and a pair of pads in a braking operation.